Do not run if rustfmt.toml does not exist
Do not run if rustfmt.toml does not exist

distcheck (and generally publishing tarballs) will not package
rustfmt.toml and we for now still support running tidy etc in those
tarballs.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust